The present invention relates to a grease composition, and more particularly to a grease composition for preventing fretting at sliding or joint portions of parts used for constraining relative motions or parts bearing fine reciprocating motions.
There are various mechanical parts which are suffering from abrasion referred to as fretting, examples of such mechanical parts being parts for restraining relative motions, for example, shaft engagement, bolt joint, rivet joint or tapered joint, and parts bearing fine reciprocating motions, for example, ball-and-roller bearing, plain bearing, ball bush, spline shaft, flexible shaft joint, universal joint, laminated spring, coil spring, electric contact, valve and valve seat or wire rope. Particularly, for transportation of motor cars, long distance transportation has been carried out by trailers or freight trains. During such long distant transportation, rolling surfaces of bearings are suffering from fretting by fine vibration to thereby arise a problem.
Various methods have been proposed to prevent such fretting, including a method in which a properly selected lubricant is used to prevent fretting. Prevention of fretting by grease lubrication has been reported. However, different results were found for thickners as test methods are varied. Additives containing phosphates have been found to exhibit advantageous effects, but the effects are significantly affected by chemical structure of the additives. The performance characteristics of a particular grease for the prevention of fretting have not yet been sufficiently clarified.
On the other hand, we have found a urea grease composition which is improved in various properties, and patent applications were filed therefor (see Japanese Patent Publication No. 11156/1980 and Japanese Laid-open Patent Application Nos. 250097/1987 and 9296/1989).
After eager investigation, we have found that a grease composition containing a urea thickener to which there is added a specific compound is considerably improved in fretting prevention capacity as compared to the conventional grease compositions.